156 ON THE COLOURS OF THICK PLATES. rings, the Duke de Chaulnes* discovered accidentally that their brilliancy was greatly increased by breathing on the glass. Since the moisture soon evaporated, in order to procure a permanent tarnish, he spread over the surface a small quantity of a mixture of rnilk and water, which on drying left a degree of dimness very suitable to the experiments. By substituting for the glass mirror a metallic speculum, in front of which there was placed a plate of tarnished mica, it was easy to observe the variation in the diameter of the rings corresponding to a variation in the distance of the mica from the speculum. In this form of the experiment the glass plate was replaced by the plate of air comprised between the mica and the speculum. Rings were also produced when the tarnished mica was replaced by a screen of fine muslin. In this case, however, according to the Duke's statement, the rings were nearly square, though rounded off a little at the angles. A set of parallel wires gave merely a bright band intersected by short bands which were vividly coloured. Even the blade of a knife produced a similar appearance, weak indeed, but sufficient to establish the identity of the effect. It is unnecessary here to discuss the theoretical views of the Duke de Chaulnes, since the progress of optical .science has since led to a complete explanation of the formation of the rin^s. The colours of thick plates were first explained on the un-dulatory theory by Dr Young f, by whom they were attributed to the interference of two streams of light, of which one is scattered on entering the glass, and then regularly reflected awl refracted, and the other regularly refracted and reileeted, and then scattered on its return through the first surface!. I)r Young's explanation is however excessively brief; and ho has rather pointed out the application of the grand and newly-discovered principle of interference to the explanation of the phenomenon, than followed the subject into any of its details. At the same time, it appears evident, from an attentive perusal of what he 1ms written, that at least the broad outlines of the complete explanation were clearly present to his mind. In the course of a paper entitled " Kxperiments for investi<>'atino-the cause of the coloured concentric rings, discovered by Sir Isaac, * Memoires de V Academic., 1755, p. 1;-J(>. t On the Theory of Light and Colour*. r for IS02, p. 41.